1. Field of the Invention
The present invention relates to an array antenna apparatus allowing electrical switching of directivity.
2. Description of the Background Art
A conventional array antenna apparatus has a two-dimensional structure shown in FIG. 11, for example (see “Basic Theory on 2-element Espar Antennas from Reactance Diversity Viewpoint”, Ohira, Iigusa and Taromaru, Technical Report of IEICE, AP2002-93, pp. 13–18). A conventional array antenna apparatus 100 includes a dielectric substrate 110, a feeder element 111 arranged on one main surface of dielectric substrate 110, and parasitic elements 112, 113.
Dielectric substrate 110 has a substantially rectangular two-dimensional shape, and feeder element 111 and parasitic elements 112, 113 are arranged in parallel to one side of the rectangle.
More specifically, parasitic elements 112 and 113 are arranged symmetrically around feeder element 111. Intervals d between feeder element 111 and parasitic element 112 and between feeder element 111 and parasitic element 113 are set to λ/4 or λ/10, when a radio wave transmitted/received by array antenna apparatus 100 has a wavelength of λ.
Parasitic elements 112, 113 have varactor diodes 114, 115 serving as variable capacitance elements loaded, respectively. By controlling a voltage supplied to varactor diodes 114, 115, array antenna apparatus 100 has its directivity switched, while maintaining impedance matching. More specifically, when voltages supplied to varactor diodes 114, 115 are denoted as V1 and V2 respectively that can be set to Va, Vb respectively, voltages V1 and V2 are switched between [V1=Va, V2=Vb] and [V1=Vb, V2=Va]. Then, reactance values −Xa, −Xb loaded to parasitic elements 112, 113 respectively are switched, so that array antenna apparatus 100 has its directivity switched, while maintaining impedance matching.
FIG. 12 illustrates a directivity gain pattern in a plane provided with an antenna, that is, in a φ plane, when interval d is set to λ/4, while FIG. 13 illustrates a directivity gain pattern in the φ plane when interval d is set to λ/10. When interval d is set to λ/4 and when a set of reactance values −Xa, −Xb is set to [−Xa=−455Ω, −Xb=−37Ω], array antenna apparatus 100 shows a directivity gain pattern PT1, and attains high gain in a direction where φ=270°. Meanwhile, when a set of reactance values −Xa, −Xb is set to [−Xa=−37Ω, −Xb=−455Ω], array antenna apparatus 100 shows a directivity gain pattern PT2, and attains high gain in a direction where φ=90°.
When interval d is set to λ/10 and when a set of reactance values −Xa, −Xb is set to [−Xa=−455Ω, −Xb=−37Ω], array antenna apparatus 100 shows a directivity gain pattern PT3, and attains high gain in a direction where φ=270°. Meanwhile, when a set of reactance values −Xa, −Xb is set to [−Xa=−37Ω, −Xb=−455Ω], array antenna apparatus 100 shows a directivity gain pattern PT4, and attains high gain in a direction where φ=90°.
Therefore, whether interval d is set to λ/4 or λ/10, setting of a set of reactance values −Xa, −Xb is switched between [−Xa=−455Ω, −Xb=−37Ω] and [−Xa=−37Ω, −Xb=−455Ω], so that array antenna apparatus has its directivity switched between a direction of 90° and a direction of 270°. The direction of 90° and the direction of 270° correspond to a direction DR1 in FIG. 11.